Idea: Sending multiple events to a single room in a linked batch

This issue has been migrated from [#12010](https://github.com/matrix-org/synapse/issues/12010).

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I floated this idea when we were trying to shut down a room and it took a while because it has to run state resolution after sending each event.

Instead of sending each event in a sequence one-by-one, can we come up with a scheme that sends a chain of events, such that all of them (except the first) have `prev_events=[the previous one]` and then make use of this in situations where we know we need to send multiple events?

This may help for:

* batch sending (batch import of history)?
* unsure if any work has already been done here to improve performance
* shutdown rooms / mass leaves
* room creation (there are a few events to be sent at once)
* room upgrades?

and possibly others...

Edit: can't remember if there was an issue already open for this and can't dig it up, sorry if it's a duplicate.